This is the 4583rd most frequent German word.
seinerseits
"On his/her/their part" or "for his/her/their part."
The usage of 'seinerseits' here indicates 'for his part' or 'on his side', showing that he personally took action or responsibility.
Er hat seinerseits die Initiative übernommen.
He, for his part, took the initiative.
In this sentence, 'seinerseits' implies an action or decision made by him personally, emphasizing his perspective or stance in the decision.
Die Vorschläge wurden seinerseits abgelehnt.
The proposals were rejected by him.
Here, 'ihrerseits' (the feminine form), denotes 'for her part', indicating her individual contribution to the action described.